Yeah, the original "300" was flawed. Not solely because of the percentages, but because of the amount of iron itself you can get out of blood being over estimated by a factor of about 10. Somebody actually did an experiment to find out how much iron you can get from blood, there used to be a video on YouTube of it, and the amounts were way less than the original 300 estimate.
Depends on the grade of steel. If you want stainless steel or chromoly steel, you'll need more nickel, chromium, and molybdenum. Otherwise you can get a mild steel with just the iron, carbon, and trace metals in human blood.
For bonus points, drain a few dozen more folks and use blood serum as your quenching oil to go full fucked up.
Instead of figuring out how to build the enterprise irl humans have decided to work on figuring out how to make a sword from the blood of their enemies